1. Field of the Invention
The present invention relates to a spread spectrum communication apparatus and method for multiplex communication using a plurality of codes.
2. Description of the Related Art
A spread spectrum communication apparatus using quadrature modulation is disclosed in U.S. Pat. No. 08/233,244 (filed on Apr. 26, 1994). In this prior art, a multiplexing method using a plurality of spread codes is described. FIG. 17 shows a transmitter, and FIG. 18 shows a receiver. In FIG. 17, transmission data is converted into parallel data #I1 to #In and #Q1 to #Qn for two channels, i.e., an in-phase channel (Ich) and a quadrature channel (Qch). Transmission data #I1 and #Q1 are spread-modulated in accordance with a common spread code PN.sub.1 which is output from a code generator 84, and then subjected to quadrature modulation. These data are converted into desired frequencies and transmitted. Similarly, transmission data #I2 to #In and #Q2 to #Qn are spread-modulated in accordance with common spread codes PN.sub.2 to PN.sub.n, respectively, and then subjected to quadrature modulation.
Referring to FIG. 18, on the reception side, the received signal is separated into the channels Ich and Qch by using a carrier which is reproduced by a carrier reproduction circuit 91, and correlations between the received signals and the spread codes PN.sub.1 to PN.sub.n which are the same as those on the transmission side are calculated by correlators 907-1 to 907-n and 908-1 to 908-n, thereby performing spread demodulation. Thereafter, a determiner 909 demodulates the data.
In the above prior art, however, the in-phase channel (Ich) data and the quadrature channel (Qch) data are spread-modulated using the common spread codes. For this reason, as in normal QPSK, a shift between quadrature axes or a phase/frequency shift in a reproduced carrier poses a serious problem. Since a highly precise quadrature and reproduced carrier are required, the apparatus can hardly be made compact, and expensive parts are necessary. Furthermore, to increase the phase/frequency precision of the reproduced carrier, the time required for carrier reproduction is prolonged. Particularly, in packet communication, the overhead becomes large, resulting in a decrease in throughput.
In the above prior art, a plurality of signals which have undergone spread modulation are linearly added and then modulated using carriers. For this reason, the amplitude variation is large, resulting in a large increase in maximum peak power with respect to the average power. In, e.g., octet-multiplexing, the maximum peak power is eight times larger (9 dB) than the average power. Therefore, the average output and communication distance become small. To ensure a predetermined communication distance, a high-power amplifier is needed, resulting in an increase in power consumption.